1. Scope Of Invention
This invention relates generally to pressure control valves in hydraulic systems, and more particularly to a pilot operated pressure valve which eliminates pressure spikes and hydraulic shock in such systems.
2. Prior Art
Pressure relief valves are used to provide a quick opening for excessive hydraulic pressure in a hydraulic system into which the valve is installed. These valves are characterized by a structure by which hydraulic pressure in the system is regulated by relieving and venting some of the pressurized fluid back to a supply tank or reservoir.
All conventional relief valves such as a differential piston relief valve or a pilot operated relief valve have a significant shortcoming when installed into a hydraulic system. When the hydraulic system is actuated so as to energize a hydraulic actuator, cylinder or motor, pressure increases virtually instantaneously. As a result, there is a sharp hydraulic spike in the pressure level of the fluid system which results in excessively abrupt energizing of the hydraulic motor. Not only is this operational limitation abusive to the system, but it may also be operationally detrimental in that the equipment being operated will exhibit too sharp a start-up.
Other patented relief valves claiming a xe2x80x9csoft startxe2x80x9d feature are disclosed in U.S. Pat. No. 4,653,527 to Kosarzecki, in U.S. Pat. No. 5,050,636 invented by Sagawa and in U.S. Pat. No. 5,381,823 invented by DiBartolo. In each of these prior art valves, pressurized fluid flow into the valve acts to move an internal piston that further loads a spring which increases the maximum operating pressure setting of the valve.
A general technical problem with the above relief valves concerns the principal embodied in each that depends upon a very high pressure drop across a pilot orifice which varies somewhat proportional to inlet pressure. At high inlet pressure and at sudden pressure increases, the setting of the valve increases much faster than at a low pressure having a slower pressure increase. In order to realize technically reasonable flows, Kosarzecki and DiBartolo both reduce the effective throttle diameter by using a wire or pin in a hole thus creating a very small ring area. However, the flow across such arrangements is viscosity sensitive.
The friction of the moveable piston in each of these devices is a source of yet another problem in loading the pilot spring to increase the operating pressure. The moveable piston must seal high pressure against the low pressure both existing simultaneously within these prior art valves. DiBartolo uses two seals that see the full pressure drop across the valve. The resulting friction drastically affects the performance of the valve. Thus, DiBartolo had to increase the effective area of the piston that loads the pilot spring. By this arrangement, the piston begins increasing the operating pressure setting of the valve at pressures much lower than the actual inlet pressure of the system. As a result, the valve is often prematurely set at a maximum setting thus having lost its damping or xe2x80x9csoft startxe2x80x9d feature altogether.
Kosarzecki reduced the friction of that valve by using a spool type piston without rubber seals. Although this device reduces the friction to a high degree, Kosarzecki still recommends an effective area for the spring-loaded piston that is ten percent (10%) greater than the effective area for the main piston. As a result, the setting of the valve is ten percent higher than the actual pressure if the inlet pressure remains steady for a period of time. Moreover, at sudden pressure increases, the Kosarzecki valve is closed first and pressure peaks cannot be eliminated. Further, this valve works only for a flow path which is xe2x80x9cside-to-nosexe2x80x9d which means that the operating pressure at the side of the valve is relieved to the nose thereof. The preferred flow path for cartridge valve is xe2x80x9cnose-to-sidexe2x80x9d for many practical reasons.
The Sagawa patent reduces the friction at the loading piston by also using a spool-type piston without rubber seals. However, Sagawa also uses a differential area for this system so that the two diameters of the piston and the spool require very accurate manufacturing and concentricity.
These prior art soft start valves are direct acting relief valves. In contrast, the present invention is a pilot operated relief valve. As in other pilot operated relief valves, the pressure in the pilot chamber is much lower than the controlled pressure, but the loading mechanism always sees much higher pressure. Since the mechanism that changes the setting of the valve in the present invention is on the pilot side of the valve, this mechanism sees much lower pressures than the existing valves. That makes it possible to change the setting of the valve slowly and with low hysteresis.
The general principal incorporated in U.S. Pat. No. 6,119,722 and U.S. Pat. No. 6,039,070 involves limiting the pilot chamber pressure and rate of inlet pressure rise. This is accomplished in large part by positioning the variable spring loader mechanism in the very low pressure in the pilot chamber just sufficient to fully bias the pilot chamber spring to its maximum pressure setting. Moreover, the operating pressure at which the present invention opens slowly follows the actual pressure at the inlet port of the valve. At sudden pressure increase in the system when the inlet pressure exceeds the maximum valve setting, the valve opens until the setting and the actual pressure are equal again. Thus, assuming the flow does not exceed the capacity of the valve, the pressure at the valve inlet cannot rise faster than the operating pressure setting of the valve itself.
The present invention provides an improvement to the above-referenced ""722 and ""070 patent mechanisms by teaching a pressure compensated priority flow control valve which more accurately and smoothly controls the movement of the pilot sleeve and modulates that movement between a threshold operating pressure and a maximum operating pressure of the valve. By providing careful controlled flow of pressurized fluid into and from the adjust chamber of the present invention from the pilot chamber, the speed of the moving pilot sleeve is more accurately controlled as it modulates the variation in operating pressure of the valve against the pilot chamber springs and pilot ball, including increasing the threshold and the maximum operating pressure values.
This invention is directed to a pilot operated pressure valve which limits the rate of start-up pressure rise (xe2x80x9csoft start-upxe2x80x9d) and substantially eliminates excessive system pressure spikes. The valve includes an inlet, a hollow cylindrical main body sealingly connected at one end thereof to the inlet, a pilot chamber having a sleeve slidably mounted within the main body and an operating pressure adjustment connected to another end of the main body adjacent another end of the pilot chamber for varying the threshold operating pressure of the valve. A main chamber receives pressurized fluid metered through a main orifice and discharges pressurized fluid through a damping orifice when the main chamber is filled and fluid inlet pressure exceeds the inlet threshold pressure which is just sufficient to open a spring biased pilot ball downstream of the damping orifice. An exit port upstream of the main orifice will open for discharging pressurized fluid from the valve back to a supply tank when inlet pressure exceeds the maximum operating pressure setting of the valve. Pressurized fluid is also metered from the pilot chamber into a pressure compensated priority flow control valve arrangement including an adjust chamber to automatically vary the operating pressure of the valve in xe2x80x9csoft start-upxe2x80x9d fashion by selectively discharging pressurized fluid from the pilot chamber and/or the adjust chamber in modulated fashion when fluid pressure reaches the threshold operating pressure.
It is therefore an object of this invention to provide a pilot operated pressure valve which genuinely exhibits a limitation of pressure spikes and a rate of pressure rise of the fluid pressure in a hydraulic system.
It is still another object of this invention to provide a pressure valve for a hydraulic system which limits the pressure within the system so as not to exceed the maximum pressure setting of the valve.
It is yet another object of this invention to provide a pressure valve which limits the rate of pressure rise in a hydraulic system up to and not exceeding the maximum pressure setting of the valve.
It is yet another object of this invention to provide a pressure valve which exhibits the above features in various embodiments such as that of a pressure relief valve, a vented relief valve, a sequence valve, and a kick-down relief valve.
Yet another object of this invention is to provide a xe2x80x9csoft startxe2x80x9d and xe2x80x9csoft reliefxe2x80x9d valve having an adjust chamber which carefully modulates the return flow of pressurized fluid from the valve between the variable threshold and maximum operating pressure of the valve.